The invention relates to a method for the selective catalytic reduction of nitrogen oxides in the exhaust gas of internal-combustion engines in motor vehicles, wherein a reducing agent is fed into the exhaust gas of the internal-combustion engine from a reducing agent tank internal to the vehicle.
From technical literature and utility vehicles, a method of this type is fairly well known, which is called the (Selective Catalytic Reduction) SCR method. By use of the known SCR method, nitrogen oxide emission is drastically reduced in the case of diesel engines and, by use of a urea solution as the most frequently used reducing agent—especially under the name AdBlue—, is converted to water vapor and nitrogen. AdBlue is carried along in motor vehicles in an additional tank (reducing agent tank) and is injected into the exhaust system. In connection with an SCR catalyst, metering devices in the exhaust system contribute to the reduction of nitrogen oxide emissions and soot particles. Since the urea solution freezes as early as at a temperature of approximately minus 12 degrees, it has to be heated at least in the area of the drain pipe in order to at least partially thaw AdBlue that may be frozen. As a result of the removal by suction of partially thawed urea solution, an ice cavity may form around the heating element and the suction point.
It is an object of the invention to improve the method of the above-mentioned type with respect to a detection and reduction of the formation of ice cavities.
According to the invention, this task is achieved by a method for the selective catalytic reduction of nitrogen oxides in the exhaust gas of internal-combustion engines in motor vehicles, wherein a reducing agent is fed into the exhaust gas of the internal-combustion engine from a reducing agent tank internal to the vehicle. An electronic control device, by way of the signal of a sensor in the reducing agent tank, detects whether the reducing agent is in a frozen state. The electronic control device applies heating current to a PTC heating element in the reducing agent tank if the reducing agent is in a frozen state. The electronic control device calculates the resistance of the PTC heating element from the onboard voltage of the motor vehicle and the heating current and compares the resistance with a first defined resistance threshold and/or with a first defined gradient threshold of the increase in resistance. The electronic control device reduces the amount of reducing agent fed into the exhaust system from the reducing agent tank if the first resistance threshold and/or the first gradient threshold is exceeded.
The invention is based on the following recognitions. The activating of the heating element takes place with the goal of heating the reducing agent. Heating increases the resistance of the heating element. The resistance therefore represents a measurement for the energy stored in the heating element, and the temperature change represents a measurement for the converted power. When the heating current is applied to the heating element, the PTC heating element will heat up and the resistance will rise. A temperature level and a resistance value will be set as a function of the energy input into the PTC heating element and of the wetting of the heating element. A resistance course is obtained when wetting degrees of the heating element differ. The course of the resistance may depend on the absolute supply of energy, the energy conductance corresponding to the wetting, the starting temperature, the time and the energy supply per time. However, in general, the final value and also the rise of the resistance will be the greater, the lower the wetting and thus the heat conduction.
According to the invention, a differentiation is therefore made between the following three operating conditions:
(1) Detection of Normal Operation (=Fully Wetted):
The resistance rises over a period of time to be defined and is adjusted such that the course of the resistance has a gradient of approximately zero. The adjusted resistance will then remain below the first resistance threshold and/or gradient threshold.
(2) Detection of Partial Wetting Operation:
If the resistance assumes a value above the first resistance threshold and/or gradient threshold, which suggests a partial wetting, but below a second resistance threshold and/or gradient threshold, which is defined for the recognition of an ice cavity—thus of an air environment that does not cool—, the carried-away reducing agent quantity will be reduced in order to permit an increased thawing of the frozen reducing agent.
(3) Detection of Ice Cavity:
If the second resistance threshold and/or gradient threshold for the ice cavity recognition is exceeded, stronger safety measures have to be taken than in the case of a partial wetting operation.
The invention not only ensures the implementability of an SCR method but also a simple diagnosis of the used components, such as the heating element or the level sensor. The reason is that the signals of the level sensor and of the resistance course of the PTC heating element can be made plausible by means of comparison when the position of the heating element with respect to the level sensor is known.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.